In general, vehicle climate control systems include a controller that regulates a number of parameters such as blower motor speed, refrigerant compressor activation and/or capacity, air mixing door position, and discharge temperature. In a manual system, the operator directly or indirectly controls the parameters, while in an automatic system, the parameters are automatically controlled in response to a number of inputs, including cabin air temperature, outside air temperature and solar loading, to regulate the cabin air temperature at a set temperature selected by the operator. In either type of system, front and rear window defogging functions are ordinarily manually activated by the operator when the presence of fogging is noticed.
The desirability of providing automatic activation of front and rear defogging functions has been recognized in the prior art. See, for example, the U.S. Pat. Nos. 4,920,755; 5,653,904; 5,701,752; and 6,311,505, the German Patent No. DE 19942286, and the Japanese Patent No. 60-248423. These are typically on/off systems that override existing control settings when fogging or the potential of fogging is detected. Additionally, the U.S. Pat. No. 6,508,408 to Kelly et al. discloses a technique for calculating a fog factor that quantifies the potential of fogging, and using the fog factor to variably bias the control settings toward values that maximize defogging. However, calculating the fog factor of Kelly et al. requires knowledge of the relative humidity and air temperature near the windshield, as well as the air temperature elsewhere in the passenger compartment, and the cost of sensors for measuring these parameters can make the system too expensive for many production vehicles. Accordingly, what is needed is an accurate and cost-effective way of characterizing windshield fogging potential, and a control method for implementing an automatic defogging control based on the fogging potential.